WASHINGTON — Sen. Bernie Sanders, one of the most liberal members of Congress, will announce on Thursday that he will seek the Democratic nomination for president, a close ally of the senator confirmed Tuesday.

Sanders will soon file paperwork with the Federal Election Commission and make a formal announcement in late May at City Hall in Burlington, where he once served as mayor, the unnamed source told USA TODAY.

He is the first well-known candidate to challenge former Secretary of State Hillary Clinton for the Democratic nomination. The progressive senator is expected to run well to the left of Clinton.

It also will be the first time that the fiercely independent politician has run as a Democrat. He has long criticized both parties for being too beholden to corporate interests. He does not accept campaign contributions from corporate political action committees.

Sanders, following a speech Tuesday night at Howard University in Washington, D.C., avoided commenting directly on the widespread reports of his impending announcement.

"Let me be diplomatic here," he said in response to a question about whether he will run and, if so, how young people could become involved in the campaign. "I promised I would make a decision by the end of this month. What's today? The 28th?

"A formal decision will be announced before the end of this month, and we would love to have you on board."

Sanders' No. 1 message, which he's promoted passionately for years, is that the gap between rich and poor is widening and the middle class is collapsing. If nothing changes, Sanders warns, a handful of billionaires will eventually control the economic and political systems.

A Sanders' adviser said income inequality will be a theme of his campaign, along with the threat of climate change and the "threat to democracy" posed by the Supreme Court's 2010 ruling in Citizens United v. the Federal Election Commission that allows corporations and unions to spend unlimited amounts of money on ads supporting or opposing individual candidates.

"The bet is that there is a huge audience for that," the adviser said.

His long-shot White House bid could shift the presidential debate to the left as he promotes progressive causes, including an expansion of Social Security and a pledge to take on "the billionaire class."

Sanders speaks at a rally at the Vermont statehouse on July 1, 2016, in Montpelier, Vt., held to commemorate the implementation of the nation's first state law to require the labeling of foods made with genetically modified ingredients. Wilson Ring, AP

Sanders leaves the Vatican after the conference commemorating the 25th anniversary of "Centesimus Annus," a high-level teaching document by Pope John Paul II on the economy and social justice at the end of the Cold War, at Vatican City on April 15, 2016. Angelo Carconi, European Pressphoto Agency

Surrounded by members of the National Nurses United in "Robin Hood" hats, Sanders participates in a news conference on Capitol Hill on May 19, 2015, to discuss college tuition legislation. Jacquelyn Martin, AP

Vermont's top political figures, from left, then-senator James Jeffords, Sanders, Sen. Patrick Leahy and then-governor Howard Dean have a toast of milk as they celebrate the passing of the Northeast Dairy Compact in Montpelier, Vt., on Nov. 22, 1999. Toby Talbot, AP

The 73-year-old Sanders, who has promised to continue rejecting contributions from corporate political action committees, faces an enormous financial challenge in making sure his campaign themes aren't eclipsed by those of former Secretary of State Hillary Rodham Clinton.

Progressives also have pledged money to entice Democratic Sen. Elizabeth Warren of Massachusetts to run, although she has said she has no plans to do so.

Sanders had $4.6 million in his Senate campaign account at the end of March that he could use in a presidential campaign. He generally receives most of his contributions from individual donors, but political action committees representing labor unions are longtime contributors. Groups supporting Clinton plan to raise hundreds of millions for her campaign.

The Sanders' adviser said Sanders will try to replicate his successful record of raising small contributions on a national scale and find new ways to engage supporters online. For instance, he will try to use technology to help supporters organize themselves on his behalf rather than micromanaging them from a national headquarters.

"The heart of this campaign as it develops and grows will be an effort to communicate with millions of people online," the adviser said. "We're going to try to take what President Obama did and move it forward."

The adviser said Sanders' headquarters will be in Vermont but he will have a strong presence in early voting states, beginning with Iowa and New Hampshire. The official launch of his campaign in Vermont will be followed by events in early voting states.

Sanders, the son of a Polish-Jewish immigrant father and a New York-born mother, is a former mayor of Burlington, Vt., and the longest-serving independent member of Congress in history.

He has caucused with Democrats since his days as Vermont's lone congressman, beginning in 1991. He believes lessons can be learned from Scandinavia's governing principles: Treat health care as a right, provide higher education for free, protect the environmental and reduce childhood poverty.

Vermont Public Radio was the first of numerous news outlets to publish stories based on anonymous sources Tuesday about Sanders' entry into the 2016 presidential race.

Mike Hersh, of Progressive Democrats of America, said Sanders will bring a populist, progressive vision to the debates and campaign.

"He has spoken out about the need to make our economics and politics work for all Americans — not just the tiny powerful and wealthy elite," Hersh wrote in an email.

Sanders has been testing his message in early primary states and the South. A key element of that message is his prediction that a "very significant rebellion" will result if the wealthiest 1 percent of Americans continue to get richer as everyone else gets poorer.

He has proposed spending $1 trillion over five years to rebuild roads, bridges and transit systems and he has called for tuition-free courses at all public colleges and universities.

He also says the budget should target corporations and wealthy Americans who avoid taxes by stashing cash offshore in places like the Cayman Islands.

"If we are serious about dealing with the massive problems facing our country, this is an issue that must be addressed and certainly addressed before we talk about cutting programs for the elderly and the children," he said in January.

Sanders has belonged to one political party in his lifetime — the anti-war Liberty Union Party back in the 1970s — and he spent a long time weighing whether to run for the White House as a Democrat or an independent.

He told USA TODAY in February that Americans are unhappy with the two-party system and believe Republicans have moved too far to the right and Democrats haven't fought hard enough for working families. But campaigning outside the two-party system requires candidates to be billionaires or to find some other way to establish a political infrastructure in all 50 states, he said.

Sanders has said previously that he wouldn't stay in the race until November and "play the role of a spoiler" if he were to run for president and the campaign didn't catch on.

Progressives have said they hope that if Clinton runs for president, she faces a primary challenger who focuses the debate on income inequality and expanding Social Security, Medicare and Medicaid.

Last spring, Progressive Democrats of America launched its "Run Bernie Run" campaign and collected several thousand signatures urging Sanders to run as a Democrat. The group has organized house parties, some featuring Sanders as a speaker, and has distributed hats, bumper stickers and buttons.

"His campaign is designed to inspire and involve millions of voters who have lost hope and stopped voting because they understand the system has failed to address their basic needs," Hersh said.

Democracy for America and MoveOn.org Political Action are behind the "Run Warren Run" campaign, a national grassroots effort to urge Warren to run for president. But Democracy for America spokesman Neil Sroka said in late February it would help to have Sanders' voice in a debate with Clinton and Warren.

"Our view is that the more voices that are involved in the Democratic primary standing up and fighting for working families, the better," he said. "The more likely we are to get a nominee who's going to be a strong champion for populist progressive values."